1. Field of the Invention
The invention relates to a lens module, more particularly to a lens module suitable for use in a thin scanning device.
2. Description of the Related Art
In recent years, scanning devices have been used quite widely. While functionality of scanning devices has increased, the trend is still to miniaturize the size thereof. The size of a scanning device depends on the size of a lens module, which serves to project an image of an original onto an image sensor, within the scanning device.
At present, a critical factor that determines the size of the lens module is the total length of an optical path and the back focal length. If the total length of the lens module or the back focal length is relatively long, the overall size of the lens module will definitely be larger. Attempts to reduce the total length so as to reduce the size of the scanning device necessitates the design of a lens module that can match the reduced total length in order to achieve a satisfactory imaging quality.
U.S. Pat. Nos. 5,386,312, 5,731,915, 6,014,262, 6,147,811 and 6,208,474 have disclosed lens modules of various types. However, due to limitations posed by optical design techniques and lens manufacturing techniques, the lens modules disclosed in the aforesaid patents are not able to meet the requirements for a reduced total length and a satisfactory imaging quality.